This invention is directed to a metal-insulator semiconductor (MIS) transistor device, and more particularly to a MIS semiconductor device comprising two MIS transistors of which the gates are electrically connected to each other, source and drain of one transistor are used as such, and the channel of another transistor as a gate.
A conventional MIS transistor has a structure as shown in FIG. 1A in which two n-type regions or islands 13 are formed in a p-type semiconductor 12, an insulating layer 14 of silicon dioxide (SiO.sub.2) is formed across these n-type regions, with a gate 11 fabricated on the insulating layer. These two n-type islands are connected to source (S) and drain (D) terminals, and the gate to the gate terminal (G). The equivalent circuit of the transistor of FIG. 1A is as illustrated in FIG. 1B.
A MIS transistor has a weakness when compared with a bipolar transistor in that the threshold voltage varies or is not uniform from one transistor to the other. It is understood that one primary cause therefor is the difference of characteristics of the phase boundary between silicon and silicon dioxide. Within the substrate of the same crystal, such a difference is due to the fact that the phase boundary is positioned apart. Where there are more than one semiconductor substrates, such a difference is caused by the fact that the substrates are independently processed. For example, alkaline ions (positive ions) of sodium or potassium may enter into the layer of silicon dioxide from outside while the insulating layer is being fabricated in the course of processing the substrate, resulting in the difference of phase boundary characteristics.
Variation or change of the threshold voltage may be understood to be the effect of an imaginary battery 15 existing between gate 11 and SiO.sub.2 insulating layer 14 with its negative side connected to the gate as illustrated by an equivalent circuit of FIG. 2, the battery supplying the voltage of which value corresponds to the change of the threshold voltage. The problems encountered in using MIS transistors are that the threshold voltage changes from one lot of MIS transistors to another, and also that it changes as time lapses.